Image projection system with variable display format

ABSTRACT

An image projector is provided with a light source and a light valve positioned downstream of the light source. An optical path is defined between the light source and the light valve. To accommodate various image viewing formats, a plurality of optical integrators is positioned between the light source and the light valve. Each of the plurality of optical integrators is movable laterally of the optical path and is positionable in the optical path. Light from the light source is transmitted to the light valve through the particular optical integrator positioned in the optical path. Each optical integrator is constructed to produce a unique display format for the light transmitted therethrough to the light valve, in order the accommodate different display formats such as standard and letter-box without wasting light.

FIELD OF THE INVENTION

The present invention pertains to an image projection system, inparticular to optical path components for projecting images of variableformat.

BACKGROUND AND SUMMARY OF THE INVENTION

Projection systems have been used for years to project motion picturesand still photographs onto display screens. More recently, presentationsusing multimedia projection systems have become popular for purposessuch as sales demonstrations, business meetings, and classroom sessions.In a common mode of operation, multimedia projection systems receiveanalog video signals from a personal computer (PC). The video signalsrepresent still, partial-, or full-motion display images of the typerendered by the PC. The analog video signals are converted into digitalvideo signals to control a digitally-driven light valve, such as areflective light valve or a transmissive liquid crystal light valve(LCD), which form display images.

A popular type of multimedia projection system is a projector thatincorporates a light source and optical path components upstream anddownstream of the light valve to project the display images onto adisplay screen. Examples such projectors include those sold under thetrademark LITEPRO® by In Focus Systems, Inc. of Wilsonville, Oreg., theassignee of the present application.

It is increasingly necessary that projectors be capable of projectingelectronic images in a variety of formats. Two of the most commonformats are the 3×4 standard screen format and the 16×9 "letterbox"format. The letterbox format is utilized in some laser-disc based videosand is proposed for use in high-definition television (HDTV).

While conventional projectors are capable of handling both standard andletterbox formats, they do so with a penalty in optical efficiency. Forexample, the optical systems of conventional projectors are usuallydesigned for standard format projection, and letterbox format projectionis accomplished by "blanking" pixels in upper and lower margin portionsof the light valve. In the case of a transmissive light valve, theblanked margins block light while the active central portion of thelight valve modulates and passes light, producing a letterbox-sizeddisplay field. In the case of a reflective light valve, the blankedmargins block light by reflecting it away from the projection path, andthe active central portion of the light valve modulates and reflectslight along the projection path, producing a letterbox-sized displayfield.

In both cases, the light blocked by the upper and lower blanked marginportions is wasted, thereby compromising projector brightness in theletterbox mode. Such decrease in projector brightness is a seriousdisadvantage. For instance, even in standard display format withoutletterbox blanking, the optical efficiency of many projectors is suchthat the projectors are only marginally bright enough to produce goodimage quality in a well-lit room. Thus, the additional loss of lightattending letterbox blanking is undesirable.

Thus, in light of these disadvantages, it is a principle object of thepresent invention to provide a projector capable of standard andletterbox format projection, without loss of brightness in the letterboxdisplay format.

It is another object of the present invention to provide a projectorwith a simple system for converting between standard and letterboxformats.

In accordance with a preferred embodiment of the present invention animage projector is provided with a light source and a light valvepositioned downstream of the light source. An optical path is definedbetween the light source and the light valve. To accommodate variousimage display formats, a plurality of optical integrators is positionedbetween the light source and the light valve. Each of the plurality ofoptical integrators is movable laterally of the optical path and ispositionable in the optical path. Light is transmitted from the lightsource to the light valve through the particular optical integratorpositioned in the optical path. Each optical integrator is constructedto produce a unique display format for light transmitted therethrough tothe light valve.

Accordingly, in one preferred embodiment of the present invention afirst and a second optical integrators are provided to accommodate bothstandard and letterbox viewing formats without waste of light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan, partially cutaway view of portion of a variableformat projection optical system according to one preferred embodimentof the present invention.

FIG. 2 is a top plan, partially cutaway view of portion of a variableformat projection optical system according to another preferredembodiment of the present invention.

FIG. 3 is a top plan, partially cutaway view of portion of a variableformat projection optical system according to another preferredembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 generally shows an image projector 10 in accordance with onepreferred embodiment of the present invention. The projector includes ahousing 12 that supports a variable format projection optical system 14.The optical system, from back to front along an optical path (in thiscase an axis) 15, includes a light source 16, a variable display formatdevice 18, a collimating optics group 20, a polarizer 22, a transmissivelight valve 24, and a telecentric-type projection lens 26. As will bediscussed, the variable display format device includes a first and asecond optical integrators 28, 29 that are selectively registered withthe optical axis 15 to selectively vary the display format of theoptical system 14.

The image projector depicted in FIG. 1 will now be described in detail,from the light source 16 forward. The light source 16 is preferably ametal halide lamp 30 housed within an elliptical mirror 32. An optionalmeniscus lens 34 may be disposed downstream of the elliptical mirror 32to boost the concentration of light focused by the elliptical mirror.

The optical integrators 28, 29 are elongated with squared-off flatupstream and downstream end surfaces 28a, 29a and 28b, 29b. The firstoptical integrator 28 has a 3×4 cross-section to provide a 3×4projection display format, and the second optical integrator 29 has a9×16 cross-section to provide a 9×16 projection display format.

The optical integrators 28, 29 preferably have a single transparentdielectric core, which in combination with cladding preferably providesa high numerical aperture (NA), permitting compact light gathering, andtotal internal reflection for efficient light transmission. The lightpipe NA is preferably matched to the NA of the ellipticalmirror/meniscus lens system 32, 34. In order to provide total internalreflection, the physical structure of the optical integrator wall is areflective optical interface, i.e. the boundary of two different opticalmaterials. The illustrated nonimaging device wall forms a dielectric(e.g. acrylic)-air boundary. The optical integrators may be constructedof various suitable materials, such as glass or acrylic with or withouta cladding. It is also to be understood that effective nonimagingdevices having other boundary types may be constructed, such as air-metal, dielectric-metal, and dielectric-dielectric.

In order to appropriately integrate the light (i.e. to provide a uniformlight intensity across the projection field), the optical integrator hasa length such that sufficient reflections of light within the opticalintegrator occur. An optical integrator of high NA permits a shorteroptical integrator by permitting increased reflections per given lengthoptical integrator.

The variable display format device 18 is situated such that the upstreamends 28a, 29a of the optical integrators 28, 29 are registerable withthe optical axis 15 at the focus of the meniscus lens 34 and ellipticalmirror 32 system. The optical integrators 28, 29 are mounted on a frame36 for selective movement lateral of the optical axis 15. The frame 36may include a pair of tracks 38 that extend perpendicular of the opticalaxis 15. As shown in FIG. 1, the tracks 38 may be grooved. The opticalintegrators 28, 29 are mounted together on a sled 40, which hasextensions 42 that are received within the grooved tracks 38, to permitsliding movement of the sled perpendicular of the optical axis 15.

In the preferred embodiment, the tracks have closed ends 44a, 44b thatprovide stops for the sled at either end of the tracks. As shown in FIG.1, the tracks 38 are dimensioned such that the first optical integrator28 is centrally registered with the optical axis 15 when the sled 40 isstopped against the second end 44b of the tracks. The second opticalintegrator 29 is centrally aligned with the optical axis 15 when thesled is stopped against the first end 44a of the tracks. An actuatorpost 46 may extend from the sled and through the projector housing 12,to permit manual selective registration of the first and second opticalintegrators 28, 29 with the optical axis.

The collimating optics group 20 is positioned along the optical axisdownstream of the variable display format device 18. The collimatingoptics group usually includes a number of positive and negative lenses.The polarizer 22 appropriately polarizes light for modulation by thetransmissive light valve 24, which preferably is of liquid crystaldisplay (LCD) type. A light valve drive electronics unit 48 containing aCPU is mounted to the frame and is electrically connected to the lightvalve 24. A power supply 50 provides appropriate electrical power forthe light source 16 and the electronics unit 48. The collimated,modulated light transmitted from the light valve 24 passes through thetelecentric projection lens 26 for projection onto a display screen.

It is also contemplated that the optical integrators may beautomatically movable to accommodate the format of the image data beingprojected. In one preferred embodiment, a source cable carrying H_(sync)and V_(sync) signal conductors 52, 54 connects a multimedia source 54(such as a VCR) to the projector 10. A mode identification counter 58,which may be a part of the drive electronics unit 48, derives thedisplay format of the VCR video data by monitoring the H_(sync) andV_(sync) signals. A format signal conductor 60 interconnects the modeidentification counter and a sled actuator 62, which may be a solenoidor other actuator type. The sled actuator 62 is connected to the sled 40for selectively moving the sled to appropriately register the opticalintegrators 28, 29 with the optical axis 15.

In operation, the mode identification counter 58 counts the number ofvertical lines in a frame of data, and the average number of 50 MHzcounter clocks in a line of data to determine the height-to-width formatof the video data. Based on the display format determined by the modeidentification counter, a format signal is sent along conductor 60 tothe sled actuator 62 to appropriately register either the first orsecond optical integrator 28, 29 with the optical axis 15.

Alternative Embodiment No. 1

FIG. 2 shows an alternative embodiment of the present invention, withreference numbers in the "100" series corresponding where appropriate toreference numbers of FIG. 1. In FIG. 2, the light valve is areflective-type light valve 124, such as a DMDT™ light valvemanufactured by Texas Instruments Incorporation of Dallas Tex. A colorwheel 164 is positioned downstream of the light source 132, with theperiphery of the color wheel registered with the optical axis 115. Thecolor wheel is transmissive, with red, green and blue (RGB) filtersprovided in alternating sectors along the periphery of the color wheel.A color wheel drive motor 166 mounted adjacent the color wheel 164selectively rotates the color wheel.

The light valve drive electronics unit 148 containing a CPU is connectedvia conductors 149, 151 to the light valve 124 and the color wheel motor142, respectively. The drive electronics unit 148 synchronizes theoperation of the light valve and color wheel during projector operation.A power supply 150 provides appropriate electrical power for the lightsource 116 and the electronics unit 148.

A variable display format device 118 similar to the device 18 of FIG. 1is positioned just downstream of the color wheel. The sled 140 of thedisplay format device 118 may be manually or automatically movable asdescribed in relation to the embodiment of FIG. 1. Referring to FIG. 2,light is transmitted through a small spot area on the color wheelperiphery and is focused at the inlet port 128a of optical integrator128. Integrated light exits the optical integrator exit port 128b, isreflected by a spherical mirror 168 onto the reflective light valve 124,and is further reflected by the reflective light valve to the entrancepupil 169 of a projection lens 170. The projection lens may be a fixedfocal length lens system as shown in FIG. 2, or alternatively a zoomlens system.

Alternative Embodiment No. 2

FIG. 3 shows an alternative embodiment of the present invention, withreference numbers in the "200" series corresponding where appropriate toreference numbers of FIG. 1. In the embodiment of FIG. 3, the opticalintegrators are fly-eye type lenses 226, 227, 228, 229. This alternativeembodiment incorporates a transmissive light valve 224 and is otherwisesimilar to the embodiment shown in FIG. 1.

The light source 216 has a spherical mirror 232 to project substantiallycollimated light. The variable display format device 218 includes pairsof fly-eye lenses 226, 227 and 228, 229. The fly-eye lenses haverectangular cross-sections that correspond to and provide for selecteddisplay formats, such as 3×4 and 16×9. The upstream lens 227, 229 ofeach pair is a plano-lenticular lens, with the planar surface facing thelight source 216. The downstream lens 226, 228 of each pair is abi-lenticular lens. The lens pairs are mounted on a sled 240 that rideson tracks 238. The sled 240 may be actuated by mechanisms such as theactuator 62 and mode identification counter 58 shown in FIG. 1.

Referring again to FIG. 3, the fly-eye lens pairs integrate thecollimated light and pass the light through an optional positive lensgroup 220, a polarizer 222, and the transmissive light valve 224. Theintegrated, polarized light is modulated by the light valve 224 andpassed to a telecentric projection lens 226 for projection on a displayscreen.

It should be apparent that the invention may be embodied in otherspecific forms without departing from its spirit or essentialcharacteristics. For instance, any of the optical integrators describedabove may be trapezoidal in cross-section, in order to offset anykeystone distortion that may arise in the optical system. Theconstruction of the variable display format device and actuator may takea variety of forms. In addition, the variable display format device maybe applied to projectors using other types of reflective andtransmissive light valves. Various other elements of the optical system,for instance the spherical mirror, may also be replaced by other opticalimaging elements. Accordingly, the described embodiments are to beconsidered in all respects only as illustrated and not restrictive andthe scope of the invention is, therefore, indicated by the appendedclaims.

What is claimed is:
 1. An image projector comprising:a light source; alight valve positioned downstream of the light source; an optical pathextending between the light source and the light valve; a plurality ofoptical integrators positioned between the light source and the lightvalve, each of the plurality of optical integrators being movablelaterally of the optical path and positionable in the optical path, eachof the optical integrator transmitting light from the light source tothe light valve when positioned in the optical path, each of the opticalintegrators being constructed to produce a unique projection displayformat as light is transmitted therethrough.
 2. The projector of claim1, wherein a first of the plurality of optical integrators has a portionwith a 3×4 cross-section.
 3. The projector of claim 1, wherein a secondof the plurality of optical integrators has a portion with a 9×16cross-section.
 4. The projector of claim 1, wherein the projector hastwo optical integrators.
 5. The projector of claim 1, wherein theoptical integrators are movably held in a track, the track beingoriented such that the optical integrators are movable laterally of theoptical path.
 6. The projector of claim 5, wherein an actuator connectedto the optical integrators is operable to automatically move one of theoptical integrators into registration with the optical path.
 7. Theprojector of claim 6, further comprising a mode identification devicefor sensing the display format of incoming electronic signals, and forsignaling the actuator to move the one optical integrator intoregistration with the optical path.
 8. The projector of claim 1, whereinthe integrators comprise elongate structures that integrate light bymeans of total internal reflection.
 9. The projector of claim 1, whereinthe integrators comprise fly-eye type integrators.
 10. The projector ofclaim 1, wherein the light valve is a transmissive light valve.
 11. Theprojector of claim 1, wherein the light valve is a reflective lightvalve.
 12. An image projector comprising:a light source; anelectronically modulated light valve positioned downstream of the lightsource; an optical path extending between the light source and the lightvalve; a first and a second optical integrators positioned between thelight source and the light valve, each of the first and second opticalintegrators being movable and positionable in the optical path, thefirst optical integrator producing a first projection display formatwhen positioned in the optical path, and the second optical integratorproducing a second projection display format when positioned in theoptical path.
 13. The projector of claim 12, wherein the firstprojection display format is 3×4.
 14. The projector of claim 12, whereinthe second projection display format is 9×16.